1. Field of the Invention
The present invention relates to a displacement control valve incorporated in variable displacement compressors that are used in vehicle air conditioners. More particularly, the present invention relates to a displacement control valve that controls the flow rate of refrigerant gas between discharge and crank chambers, and includes a mechanism for changing a set value of suction pressure at which the control valve is operable.
2. Description of the Related Art
A typical variable displacement compressor has a cam plate that is tiltably supported on a drive shaft. The inclination of the cam plate is controlled based on the difference between the pressure in a crank chamber and the pressure in cylinder bores. The stroke of each piston is varied by the inclination of the cam plate. Accordingly, the displacement of the compressor is varied and determined by the stroke of each piston. The compressor is provided with a discharge chamber and a crank chamber that are connected by a supply passage. A displacement control valve is located in the supply passage. The displacement control valve controls the flow rate of refrigerant gas from the discharge chamber to the crank chamber, thereby controlling the pressure in the crank chamber. Accordingly, the difference between the pressure in the crank chamber and the pressure in the cylinder bores is controlled by the control valve.
Japanese Unexamined Patent Publication No 3-23385, discloses such a displacement control valve used in a variable displacement compressor. As shown in FIG. 7, a control valve 101 includes a housing 102. A valve seat 103 is defined at the upper portion of the housing 102. A valve hole 104 is defined in the valve seat 103. A valve body 105 is provided on a rod 106 that extends through the valve hole 104. The valve body 105 is arranged in a high pressure chamber 109 facing the valve seat 103 to open and close the valve hole 104. The rod 106 connects the valve body 105 to a bellows 108, which is located in a low pressure chamber 107. Suction pressure Ps is introduced to the low pressure chamber 107. The bellows 108 expands and contracts in accordance with the suction pressure Ps. The high pressure chamber 109 is connected to a discharge pressure area in the compressor by a supply passage. Therefore, discharge pressure Pd is introduced to the high pressure chamber 109. An intermediate pressure chamber 110 is defined in the housing 102 between the high pressure chamber 109 and the low pressure chamber 107. The intermediate pressure chamber 110 is communicated with the high pressure chamber 109 by the valve hole 104 and is connected to the crank chamber by the supply passage.
A solenoid 111 is secured to the bottom of the housing 102. A fixed steel core 113 is provided at the upper portion of the solenoid 111. A steel plunger 112 is arranged in the solenoid 111 and moves along the axis of the plunger 112. A rod 112a is coupled to the plunger 112 and extends through the core 113. A coil 114 is wound about the plunger 112 and the fixed core 113. The top end of the rod 112a is adhered to the inner wall of the bellows 108. A spring 115 extends between the bottom end of the plunger 112 and the bottom of the solenoid 111. The spring 115 urges the plunger 112 upward. That is, the spring 115 urges the valve body 105 in a direction separating the valve body 105 from the valve seat 103 to open the valve hole 104.
An external control unit (not shown) sends electric current to the coil 114. The magnetic attractive force produced between the plunger 112 and the fixed core 113 is varied by the magnitude of the current from the external control unit. The magnitude of the force that pushes the plunger 112 upward, or the force for separating the valve body 105 from the valve seat 103, corresponds to the magnitude of the attraction force. When the solenoid 111 is excited, the higher suction pressure Ps contracts the bellows 108 and lowers the plunger 112. This causes the valve body 105 to close the valve hole 104. Contrarily, a lower suction pressure Ps expands the bellows 108 and lifts the valve body 105. This opens the valve hole 104. In this manner, the opening area between the valve body 105 and the valve hole 104 is adjusted in accordance with the suction pressure Ps. A magnitude of the suction pressure Ps required for lowering the valve body 105, that is for moving the valve body 105 toward the valve seat 103, is varied in accordance with the attraction force produced between the armature 112 and the retainer 113.
The above described prior art control valve 101 has the following disadvantages.
A compressor mounted on a vehicle is connected to an external refrigerant circuit that includes a condenser. If the vehicle is caught in a traffic jam in summer, the heat exchange capacity of the condenser is significantly lowered. In this case, the valve body 105 closes the valve hole 104, and the displacement of the compressor becomes maximum. The discharge pressure Pd thus becomes extremely high, and the pressure Pc in the crank chamber approaches the lower suction pressure Ps. The high discharge pressure Pd acts on the top surface of the valve body 105. The pressure in the intermediate pressure chamber 110, or the pressure Pc in the crank chamber, acts on the bottom surface of the valve body 105. The difference between the pressures Pd and Pc strongly presses the valve body 105 against the valve seat 103. This degrades the responsiveness of the valve body 105 with respect to the suction pressure Ps.
If the cooling load falls when the displacement of the compressor is maximum, the displacement of the compressor must be decreased. In order to decrease the compressor's displacement in such a state, the opening area between the valve body 105 and the valve hole 104 must be enlarged. The valve body 105 must thus be moved by a force that is greater than the difference between the discharge pressure Pd and the pressure Pc in the crank chamber. That is, the attraction force produced between the plunger 112 and the fixed core 113 must be increased for enlarging the opening area between the valve body 105 and the valve hole 104. This requires a larger solenoid 111. A large solenoid 111 consumes a relatively large amount of power, and thus increases the load on the alternator.